The present invention generally relates to laser-equipped machine tools, and more particularly relates to real time control of laser beam characteristics for improved machine tool performance.
Laser-equipped machine tools are often used to cut parts from sheet metal and relatively thin plate. They are also used to weld together cut and machined parts. In such machine tools, a laser beam is employed to process the material. A laser beam, also simply referred to as a beam, is directed along a beam path via a beam delivery system. A beam delivery system is a collection of optical elements, such as reflective mirrors and transmissive optics, which may redirect the beam, alter the propagation characteristics of the beam or focus the beam. The beam delivery system is enclosed for safety and for control of the beam path environment within. The laser beam is concentrated by a focusing lens or mirror to a small diameter spot, which is directed to an appropriate position relative to the surface of the material to be processed.
In most implementations, the laser beam exits the laser through an output coupler, a partially transmissive and partially reflective optical element which seals the laser cavity and transmits a portion of the beam out of the laser cavity or resonator. The beam is then directed along a beam path to a focusing optic in a processing head near the work. In most cutting applications, the beam is directed by the focusing optic through a nozzle disposed immediately above the workpiece to be cut. A pressurized gas is also directed through the nozzle, typically coaxial to the beam, to assist the cutting process The pressurized gas serves to facilitate and/or shield the cutting process, and creates a gas stream which helps remove vaporized and molten material from the cut or kerf. Kerf refers to the zone of material which is acted upon and removed by a cutting process. Kerf width refers to the width of the slot created by the cutting process, such as the width of the slot cut by a laser beam as it moves along a path.
Key factors in laser processing include the diameter of the focus spot and the position of the focus relative to the material to be processed. The control of these focal characteristics is critical to maintaining the quality of the process. During processing, unintended deviation in the focus size and position may produce a deterioration in process quality and may even cause the process to fail.
The first of two main factors which influence the focus characteristics is the diameter of the laser beam at the focal optic. Due to diffraction, the minimum focal spot diameter, for a given focal length optic, is limited. Diffraction causes light beams to diverge or spread transversely as they propagate. As the input laser beam diameter increases for a given focal optic, the focus spot diameter decreases due to a decrease in diffraction. In addition, as the input laser beam diameter increases for a given focal optic, the focus spot position shifts closer to the focus optic.
The raw laser beam, issuing from the laser resonator, exhibits the characteristic of divergence. The beam diameter will change as a function of the distance from the output coupler. Typically, as the processing head moves over the processing area the distance from the output coupler to the focal optic will change. When a large processing area is required, some method of maintaining the proper beam diameter must be employed in order to avoid significant changes in focus diameter and position.
Additionally, changes in the output power level of the laser will affect the divergence of the output beam. The largest effect on beam divergence comes from the thermal loading of the output coupler which produces thermal lensing. Thermal lensing is distortion of an optical component caused by heat absorbed from the input beam. The absorbed portion of the beam causes expansion of the output coupler such that the curvature of the surface changes. The expansion causes a change in the divergence of the output beam thereby changing the beam size at any given distance from the output coupler. The rate and amount of distortion is dependent upon the power of the beam, optic contamination, thermal conductivity of the optic and its cooling system and the length of time the beam is applied. Upon reaching thermal equilibrium, when absorbed heat is in balance with that removed by the lens cooling system, the shape of the optic surface remains constant. When the beam is turned off, the optic surface gradually relaxes and returns to its original shape. When a high output power laser is required, some method of maintaining the proper beam diameter, in a time dependent response to output power changes, must be employed if significant changes in focus diameter and position are to be avoided.
The second of two main factors which influence the focus characteristics is the distortion of the focus optic due to heat absorption. In a manner similar to that described for the laser output coupler, thermal lensing occurs in the focus optic. The expansion of the focus optic reduces the effective radius of curvature which causes the focal spot to shift closer to the focal optic. When a high output power laser is required, some method of maintaining the proper focal position, in a time dependent response to input laser power changes, must be employed if significant changes in focus position are to be avoided.
Laser-equipped machine tools are typically Computer Numerically Controlled (CNC) and are manufactured in many configurations and sizes and with lasers of various types and power. Generally speaking, there are two beam delivery configurations utilized: those with a fixed length between the laser output coupler and the processing head and those with a variable path length between the laser output coupler and the processing head.
In one cutting machine configuration, typically called xe2x80x9cflying optics,xe2x80x9d the cutting head is adapted for movement along one axis, such as the Y-axis, which is mounted on a bridge adapted for movement in an orthogonal X-axis. The work is supported on a stationary pallet or table below the bridge. Movement of the cutting head is coordinated with movement of the bridge to define a precise path on the part. The cutting head and laser are controlled to pierce and cut the material, to form holes and shapes in the material and to cut the part from the material. Such machines can be configured with either a fixed length or a variable length beam path.
In a cutting machine configured with flying optics, a fixed length beam path is typically created in one of two ways. In one method, the beam path between the output coupler and the processing head consists of sections of tubular arms. The arm sections are connected via pivotable joints containing preloaded bearings with mirrors at the entrance and exit to steer the beam. As the process head moves, the tubular sections translate and pivot about the joints to follow the motion. While the fixed beam path length of such a system eliminates divergence problems due to path length, there remain concerns about the ability of the system to withstand high acceleration forces. Such a system also poses some difficulty with regard to adequately supporting the arms.
Another fixed length beam path approach is to provide an additional axis within the beam path and coordinate its movement to compensate for the positioning of the cutting head such that the length of the beam path does not change. One control means for such a system is disclosed in Fanuc Ltd. U.S. Pat. No. 5,406,048. Other methods are also in use.
On some machines, such as a xe2x80x9cgantryxe2x80x9d cutting machine, in which the laser is carried, this fixed length concept is relatively easy to implement. The machine consists of floor-mounted rails or ways about two parallel sides of a fixed table which supports the work. The rails carry a platform on which the laser is mounted. The rails also carry a gantry or bridging section over the work. Typically the laser-mounting platform is located over one of the rails such that the beam exits the laser parallel to the rails. The beam is directed by a mirror to a mirror mounting platform at the far side of the gantry beyond the cutting head. That platform has typically two mirrors mounted so as to direct the beam back to another mirror mounted on the cutting head directly over the focusing lens. The cutting head is at its closest position relative to the mirror mounting platform when the head is at its extreme travel position toward the far side of the gantry away from the laser output coupler. Movement of the cutting head on the gantry is coordinated with movement of the gantry on the rails. As the cutting head moves on the gantry, the platform with the two mirrors is coordinated to move with it, but travels half the distance of the cutting head. When the cutting head is at it""s extreme travel position nearest to the laser, the platform carrying the two turning mirrors has traveled to the approximate center of the gantry. In such manner the total length of the beam path does not change. This compensation means is often referred to as a xe2x80x9ctrombonexe2x80x9d due to the similarity of the shape of the beam path and the compensating movement to the shape and movement of the musical instrument. Such a system is difficult to implement on a flying optic machine as it is difficult to locate the trombone elements such that they do not interfere with other machine elements or with other functions such as loading and unloading material. The trombone optics must travel a greater distance on a flying optic machine as both the X-axis and Y-axis motion must be compensated. Supplying the mirror mounting platform, ways, a servo controlled drive system and machining mounting surfaces add significantly to the cost of such a machine.
One method employed to reduce the divergence of the laser beam is to expand or magnify it with a collimator. The rate of divergence of a beam is reduced in inverse proportion to the amount it is magnified. If a beam is magnified by 125 percent its rate of divergence is reduced 20 percent. If it is magnified by 200 percent its rate of divergence is reduced by 50 percent.
Collimators are optical devices, also known as beam expanders and condensers. Such devices also have other characteristics and functions known to those skilled in the art. Manufacturers of laser optics publish literature providing information on design variations and examples of use. One example of such literature is the II-IV Incorporated publication, Beam Expander-Condensers, published March 1992. Collimators can be constructed of transmissive optics such that the beam is passed through the optics. Such collimators are commonly used in laser-equipped machines up to about three kilowatt power levels and sometimes above.
Collimators used on low powered lasers are designed or adjusted to magnify the beam a given amount and then locked in place. Use of transmissive collimators with lasers having power levels above three kilowatts becomes increasingly problematic due to thermal lensing and due to limits on the energy density that transmissive optic materials can withstand. Impurities within optical materials, crystal growth conditions, surface contamination and surface imperfections are primary causes for a portion of a laser beam to be absorbed and converted to heat within a transmissive optical element.
The distortion produced by thermal lensing can influence the divergence and mode quality of the beam passing through or reflecting off of the optical delivery and focusing components and thereby cause detrimental shifts of focus position. Thermal lensing is a greater problem with transmissive optics. For example, when a high power beam is directed at the curved surface of a plano-convex focal lens, which has a curved first surface and a flat second surface, the absorbed portion of the beam causes expansion of the lens such that the curvature of the surface changes. The expansion reduces the effective radius of curvature which causes the focal spot to shift upward or closer to the lens. The rate of curvature change is greater toward the center of the lens due to the power distribution of the incident laser beam. Therefore, the heating and the expansion is greater toward the center of the lens. Fixed collimators constructed of transmissive optics are very susceptible to thermal lensing which reduces their effectiveness for use with high power lasers.
Collimators are also constructed of reflective optics, combinations of flat and shaped mirrors, such that the light beam is reflected from the optical elements. Reflective optical elements are typically manufactured from materials, such as copper, which can withstand greater energy densities without damage. Also, thermal lensing is not as severe in reflective optics as compared to transmissive optics. Thus reflective collimators are more suitably used in high power laser applications. However, a fixed, reflective collimator cannot compensate for the thermal lensing of a laser output coupler nor for the thermal lensing of a focal optic.
In view of the foregoing, it is a general aim of the invention to provide a high power laser-equipped machine tool having real time compensation of the beam size at the focusing optic and the position of the focal spot in relation to the surface of the material processed.
Stated differently, a general aim of the present invention is to provide a control system for a high power laser-equipped machine tool which is capable of adjusting optical elements in real time to maintain the focal spot size and desired position in relation to the material being processed.
A specific object of the invention is to provide a control system in which the control is real time and compensates for changes due to thermal lensing and for changes in beam path length.
In greater detail, it is an object of the invention to provide a control system for use with a servo driven precision collimator in a machine tool equipped with a high power laser, the system being adapted to introduce collimator corrections compensating for both path length changes and the thermal loading of system optics.
Stated more broadly, an object of the present invention is to provide a laser system for a machine tool in which system repeatability is enhanced by automatically, and without operator intervention, compensating for the effects on the focal spot size and location relative to the work which are introduced by beam path length changes and by the amount of power on the optical elements and the duration or frequency it is on or off, such that compensating corrections are made in real time during system operation.
Another object of the invention is to provide a real time control system for a high power laser-equipped machine tool having a transmissive focusing optic, the system being capable of compensating for changes in beam characteristics by way of a collimator and for changes introduced by the focusing optic by adjustment of the focusing optic.
It is another object of the invention to provide a control system for a high power laser-equipped machine tool having means to determine the power on the transmissive focusing optic, consider the position of the transmissive focusing optic and to respond in real time to correct the position of the lens to compensate for diffraction in and thermal lensing of the focal lens.
Another specific object of the invention according to a preferred implementation is to provide a real time control system for a high power laser-equipped machine tool having a variable length beam path and a transmissive focusing optic, to compensate for variations in the beam characteristics by using a servo controlled collimator and to compensate for changes introduced by the focusing optic by adjusting the focusing optic.
Thus, it is an objective to have the respective compensating mechanisms working in tandem to produce a consistent and repeatable beam focus spot size and position.
It is a feature of the invention that a real time control system is provided for a high power laser-equipped machine tool of the type having a variable length beam path, the system being capable of maintaining the size and diametrical characteristics of a laser beam at a focal lens, and also for maintaining the position of a focal spot in relation to the surface of the material processed.
It is a further feature of the invention that in real time separate integrators follow the thermal loading of the laser output coupler and of the focal lens with respective time constants associated with the integrators which match the thermal distortion and relaxation rate of the respective optic and that compensation values are determined from the integrator outputs and introduced into the respective drive system of the collimator and focal positioning system such that the size and position of the focal spot relative to the work processed is accurately maintained regardless of the position of the processing head within its range of travel and regardless of the amount of laser power on the optics and regardless of the duration or frequency that the beam is on or off.
It is advantageous that such a system utilize a servo controlled reflective collimator. Such a collimator is disclosed and claimed in commonly owned co-pending application Ser. No. 09/353,936, in the name of Ira E. Cole III entitled Reflective Laser Collimator.
It is also advantageous that such a system utilize a cutting head which is relatively light in weight and easily maneuverable, has a servo controlled drive to position the cutting nozzle appropriately relative to the work, has a separate servo controlled vertical adjustment drive to position its optic carrier with focusing optic and which includes a counterbalancing system, balancing the reactive forces directed on a focal optic by high pressure assist gas. Such a cutting head is disclosed and claimed in commonly owned co-pending application Ser. No. 09/302,279, in the name of John C. Legge entitled Laser-Equipped Machine Tool Cutting Head with Pressurized Counterbalance.
These and other objectives and features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.